In rotating magnetic disk memories, the recording density (bits or magnetic flux reversals per distance on a track) is limited by the inside track since it has the least circumference. Data bits are retrieved by detecting the time at which an induced peak is sensed by a read head in relation to preceding peaks. The time position of peaks is the most stable parameter under conditions of various data patterns, and is, therefore, used in preference to amplitude which varies greatly. A widely employed technique to detect the location of a peak is to take the derivative of the amplified signal. When a peak occurs, the slope will change sign and go through zero. The derivative is obtained by an electronic differentiator and when its output goes to zero, an indication of the presence of a peak can be generated. For reasons of system simplicity, it is desirable to keep the recording rate constant regardless of track position.
Therefore, magnetic transitions on outer tracks will be more widely separated than those on inner tracks because of their greater circumference. This can lead to a condition known as shouldering, which means that the read back signal does not travel smoothly from one peak to another. Between peaks, the slope assumes a relatively low value, and in severe shouldering, may actually go to zero. With a differentiator technique, these shoulders between peaks would be falsely identified as peaks.
In a real application, the nominal slope need not drop completely to zero for false peak detection to occur. White noise, impulse noise, and cross-talk from other circuits can cause sufficient deterioration to produce false detection.
A prior art method of antishouldering set an amplitude threshold on both sides of zero. This technique required that the amplified read signal traverse both thresholds for a valid peak to be recognized. The difficulty with this technique is in properly selecting the thresholds. The signal amplitude is affected by manufacturing tolerances between heads, by track position, and by recorded patterns. With typical variations, it is difficult to adequately guard against false peak detection with large signals yet not falsely reject valid peaks of small signals.